The present invention relates to a laser illumination system and a method for eliminating laser speckles thereof, especially to a laser illumination system with at least one diffractive optical element and a method for eliminating laser speckles thereof by superposition of light energy. The diffractive optical element is disposed on a pathway of a laser beam being emitted to an area to be illuminated. Thus laser beams emitted have changes in spatial phase redistribution or light energy distribution while being passed through the diffractive optical element. One point in the area to be illuminated shows energy of partial light of at least two of the laser beams and superposition of the energy of the partial light within the reaction time of the image detecting unit. Therefore the laser speckle of the image of the area to be illuminated captured by the image detecting unit can be eliminated by the superposition of energy of the partial light of the laser beams.
Laser beam, a high energy light beam, is coherent because of the same wavelength, phase, frequency and direction of propagation. Laser speckle occurs while the laser is used as a light source in a projector. A coherent light source is emitted to a rough surface such as a projection screen or a translucent diffuser plate. The light intensity distribution of the projected image is irregular due to constructive or destructive interference of scattered light at different points of the rough surface. The higher light intensity is resulted from constructive interference while the lower light intensity is resulted from destructive interference. In fact, the laser speckle is a noise that is detected by an image detection unit (such as human eyes or lenses) and is degrading definition and resolution of the image detected. The viewing comfort is affected once the image detection unit is human eyes. Thus the speckle problem of the laser illumination system should be solved.
There are certain solutions for solving the problems of laser speckles available now. For example, a projection screen is added with an actuator such as motor used for driving the screen to keep on moving or rotating. Thus coherence of the laser beams is destroyed so as to reduce the interference. Or a lens is used to focus an image generated by an image generator module onto a moveable or rotatable time-varying diffuser for destroying coherence of laser light and reducing speckles on the image. The diffuser uses high frequency vibration that results in disordered phase difference to make the observers' eyes unable to see contrast between light and dark areas of the light spots. Thus the speckle is reduced or even eliminated. However, the above devices or method use the actuator for driving the screen or the time-varying diffuser to move or rotate. The whole structure is more complicated so that not only the production cost is raised, the volume is also increased.
The quantum efficiency of laser is hundreds or thousands of times than that of LED while being applied to illumination systems. The laser has features of smaller volume, less heat generated and higher directionality so that Nobel Prize winner Shuji Nakamura believes that the LED will be replaced by laser in the next decade. However, the laser illumination technique has the following challenges in practice. First the laser produces light in a very narrow wavelength band so that there is no white laser. The second the laser light is highly directional and this has generated safety concerns. The third laser speckle resulted from coherent interference of laser light makes the observer feel uncomfortable, reduces the visibility or even severely limits resolution. In order to solve the above problems of laser, Shuji Nakamura and OSRAM project laser light onto a phosphor coating for generating on-coherent white light with low directionality. Yet the energy conversion between the laser light and the phosphor coating with different wavelength reduces the quantum efficiency of the illumination system. The light generated by phosphor has no directionality. Once the light is passed through the reflection mirror and the lenses for correcting energy distribution and matching requirements of the respective illumination or application of the display, the energy efficiency is significantly decreased and the system volume is increased. On the other hand, the laser light can be used as an ideal light source once the above three problems have been solved and no phosphor conversion is needed owing to its advantages such as the high quantum efficiency, compact volume, high directionality, long detection distance and high visibility. Thus the laser light can be applied to not only white-light illumination but also a night vision system or machine vision. Moreover, the light source of the present system can not only be applied to various lights such as headlights, desk lamps, street lights, etc., but also display systems that need an additional light source such as LCD (liquid crystal display) LCoS (Liquid crystal on silicon), etc. Now the applications of the white-light laser are around the corner. Thus the present invention focus on the design of the laser illumination system and how to solve the problems of laser speckles.